Optical inspection (AOI) systems are used for inspecting the defects of circuit; the system can be used for inspecting proper component presence and orientation, proper solder joint formation, conducting line and unwanted residue, for example photo resist residue.
As the line width of a circuit is getting narrower, it is getting easier to make mistake when doing inspection by human eye. Traditionally, to find the defect of a circuit need the experience accumulated and applying cause and effect Diagram to find out the problem; it is not only time consuming and also hard to find qualified personnel to do the job. Therefore, automated optical inspection has become popular for circuit inspection technique is the past few years.
The main automated inspection methods include inspecting by white light and by laser beams. The inspection method by white light is projecting white light on the circuit and then sensing the reflecting light signal by light sensor; at last the receiving signal is treated by the image processing software for defect identification. Another method is projecting laser beam to excite the organic fluorescence material of non metallic part and then receiving laser induced fluorescence by light sensor. By predetermined value, the metal part and non metal part of the receiving grey-scale image can be resolved; therefore the image can be used for defect inspection.
Most of the invention relates to automated optical inspection is endeavor in developing different lighting modes, signal detecting modes or other simulated methods. For example, U.S. Pat. No. 7,075,565 discloses an automated optical inspection system includes a plurality of asynchronously cameras for providing image data of a printed circuit board. The circuit board is divided into fields of view that are to be imaged in one or more cameras in one or more lighting modes. The system allows the full bandwidth of the cameras to be utilized for reducing the inspection time of the board. Taiwan Patent No. I244359 discloses a method for inspection the defect of a printed circuit board by establishing standard component templates in advance and then take these templates for defect inspection. Taiwan Patent No. I258583 discloses a lighting system for generating light with high and uniform luminescence in a small area to enhance the image resolution.
From the prior art, referring to FIG. 1, the contrast of the development and un-development area is very low. The user can not distinguish the developed area from each other. Therefore, it is unlikely to inspect the result by the operator.
Therefore, the present invention provides a method for circuit inspection, more particularly relates to a method for circuit inspection by increasing the contrast between developing and no developing area; the present method can provide a circuit image with enhanced contrast to overcome the shortcomings of prior arts.